Question

Which one of the following dyes forms a covalent bond with the fibres?

• A. Reactive
• B. Direct
• C. Vat
• D. Disperse

Hint:

Dye-Fibre Bonding:
Reactive Dyes $\rightarrow$ Covalent bonds (highest wash fastness).
Acid Dyes $\rightarrow$ Ionic Salt bonds.
Direct Dyes $\rightarrow$ Hydrogen bonds.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Dye chemistry classifies textile colorants according to the chemical nature of their bonding mechanism with polymer functional groups.
Key Formula or Approach:
\[ \text{Dye-X} + \text{Cellulose-OH} \xrightarrow{\text{Alkali}} \text{Dye-O-Cellulose} + \text{HX} \quad \text{(Covalent Bond)} \]

Step 2: Detailed Explanation:
Reactive Dyes possess reactive electrophilic functional groups (e.g., triazine, vinyl sulfone) that react chemically with the nucleophilic hydroxyl ($-OH$) groups of cellulose or amino ($-NH_2$) groups of protein fibres under alkaline fixation conditions to form permanent, irreversible covalent bonds.
Direct and Vat dyes bind via hydrogen bonds and physical entrapment; Disperse dyes form solid solutions.

Step 3: Final Answer:
Hence, Reactive dyes form covalent bonds with fibres, corresponding to option (A).